Modern communications systems based on optical fibers require massive data handling at both ends of the fiber at ever increasing speeds and ever decreasing costs. This data handling requires both optical systems such as wavelength division multiplexing (WDM) and electronic systems. The devices used to perform these tasks are high speed electronic chips and optical devices. The integration of optical devices of typically dozens of devices per chip is not as well advanced as the integration of electronic devices, where millions of devices can be disposed per chip. High optical losses within optical devices prevent the cascading of such devices and results in a small number of devices on a chip. The difficulties inherent in wavelength scale fabrication of optical devices results in low yields and high costs.
Connecting optical fiber to integrated circuits can be accomplished through various types of butt connections to the edge or surface of an integrated circuit. The butt of a fiber can be connected to a planar waveguide at the edge of an integrated circuit, but this technique is most useful if the cross sectional areas of the fiber core and the waveguide are of similar size. An optical fiber with a larger cross sectional area can be connected to a waveguide with a much smaller cross sectional area if the butt of the fiber is connected to a diffraction grating, such as a waveguide grating coupler disposed on the integrated circuit. The waveguide output of the waveguide grating coupler can be reduced to a desired smaller cross sectional area through a spot size reducer. But the packaging of such integrated circuits with optical fibers connected at roughly right angles to the surface of the integrated circuit is not very practical, since such configurations waste a great deal of space and are easy to break or damage.
One of the key achievements needed to improve the integration of optical devices is to efficiently couple light from an optical fiber to an integrated circuit and vice versa.